An insulated-gated field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located within a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
Typically within a semiconductor device, such as microprocessor for a computer, there are literally thousands of IGFETs. Designing such devices, therefore, is a complicated process, and requires the utmost care and sophistication. To assist in testing, a device may have fabricated therein a test structure. For example, a common test structure used to determine circuit speed is the ring oscillator. A ring oscillator is an odd number of inverters connected serially, with the output of the last stage connected back into the input of the first stage. When powered, this circuit starts to oscillate as a result of it driving itself. Because the output drives the input of this circuit, the oscillation speed is the highest possible for a given design.
A disadvantage to prior art ring oscillators is that the ring oscillator's speed gives only a qualitative indication of circuit speed, not a quantitative measure of speed. To more accurately determine true performance, circuit designers require electrical parameters for the individual transistors making up the circuit to be measured. Therefore, prior art devices including ring oscillators usually include individual transistors fabricated at different locations on the die from the ring oscillators. The electrical parameters or characteristics of these transistors are then substituted for the characteristics of the transistors embedded within the oscillator.
However, this approach does not yield a very accurate model of the characteristics of the transistors used to make the ring oscillators. While the isolated transistors may be designed to be identical to the embedded transistors, external variables cannot easily be taken into account. For example, the embedded transistors are typically packed very dense, and this density may have effects on the electrical characteristics of the embedded transistors within an oscillator that cannot easily be duplicated on the isolated transistors falsely assumed to be identical to the embedded transistors. There is a need, therefore, for better measurement of the electrical characteristics of the individual transistors making up a ring oscillator test structure.